The field of the invention relates generally to a vehicle time based management system, and more specifically, to a method and systems for vertical navigation using time-of-arrival control.
Conventionally, aircraft are controlled in three dimensions; latitude, longitude, and altitude. More recently, the ability to control aircraft in the fourth dimension, time, has been shown to enable advanced airspace management resulting in increased capacity. The use of time-based arrival management facilitates earlier landing time assignments and more efficient use of the runway. This also results in economic benefits if each aircraft can determine its desired landing time using its most fuel optimum flight profile. However, in the absence of a defined geometric descent profile current vertical navigation control algorithms use laws that control the elevators to a predetermined vertical path or vertical speed while maintaining a fixed throttle setting (typically idle). Using this control method the speed is allowed to fluctuate over a large range of values, resulting in varying and inaccurate Estimated Time-of-Arrivals (ETAs) at points downstream of the aircraft. This adversely impacts the aircraft's adherence to a time constraint, typically referred to as a Required Time-of-Arrival (RTA) or Controlled Time-of-Arrival (CTA).
An aircraft descent trajectory is typically constructed by an onboard Flight Management System (FMS) backward from the destination to the point where the descent begins—referred to as the Top of Descent (T/D). The vertical portion of this computed trajectory consists of three general portions:
1) Approach Segment—this is the lowest portion of the descent, and contains a deceleration to the final landing speed along with extensions of high-lift devices and landing gear.
2) Geometric Segment—this is the middle portion of the descent, and is computed as a geometric sequence of lines which attempt to honor all altitude constraints. This segment may not exist if there are no altitude constraints that require it.
3) Idle Segment—this is the upper portion of the descent, and is computed assuming the descent target speed and idle thrust. Estimated (“forecast”) winds and temperatures are assumed in the computation of this segment.
When the aircraft is flying the idle segment of the trajectory, the throttle is fixed at an idle setting and an algorithm controls the elevators to the predefined vertical path guidance mode (VPATH). However, because estimated parameters (most notably winds and temperatures) are used in the computation of the vertical path, the speed of the aircraft will vary from the target speed used in the path computation if these estimated parameters are different than the actual values encountered.
A traditional vertical navigation strategy permits the actual airspeed to deviate from the target airspeed by some preset value (a typical value is 15 knots) before either raising the throttle setting (for actual airspeed below the target) or adding drag (either automatically or by prompting the flight crew) to zero the difference between actual airspeed and target airspeed. However, using such a large tolerance around the target speed before correcting the error makes a time constraint ahead of the aircraft very difficult to meet accurately. Moreover, when the actual and target airspeeds differ by this preset value and the control strategy is changed to zero this speed error, a large amount of thrust or drag will likely be required. A known alternative vertical navigation control strategy retains the idle thrust setting and uses the elevators to control the speed as long as the actual aircraft altitude is within some range of the specified vertical path position at the current lateral position. When the actual altitude deviates by more than this value, the control strategy is modified to regain the specified vertical path while maintaining the target speed. However, this method will also have a negative affect on the time-of-arrival control if the altitude band is too large as the ground speed (which directly affects time-of-arrival) is dependant not just on airspeed but also on altitude. Conversely, if the altitude band is too small the pitch of the aircraft may be continually varying, negatively impacting the comfort of the aircraft passengers.